The present invention relates generally to convergence methods and apparatuses used in projection television (PTV) systems. More particularly, the present invention relates to a method and apparatus which enable simple, accurate and rapid adjustment of the convergence in a projection television system.
Convergence in the context of the present invention may be defined as the alignment and matching of the images produced by the image sources in a color PTV system. Typical PTV systems include, inter alia, a rear projection screen for receiving a composite image to be displayed and three separate cathode ray tubes (CRTs), each for projecting a different color component--red, green or blue--of the composite image. As the three CRTs cannot be physically located in the same place, two are usually located off the axis which is perpendicular to the plane of the projection screen (the normal). This non-alignment of the CRTs is a major source of misconvergence since the images from the three CRTs do not impinge upon the projection screen identically. Other misconvergence problems are due, for example, to inherent differences between the two or more CRTs.
To compensate for distortions caused by the non-alignment of all of the CRTs with the normal, and other distortions caused by the geometry of the screen and internal distortions of the CRTs, corrective waveforms are applied to convergence dipoles of each CRT. The coefficients of the waveforms are usually determined by separate potentiometers.
For proper convergence, prior art PTV systems require the careful adjustment of more than 30 (typically 42) potentiometers, each controlling a global aspect of the composite image such as size, overall shift, linearlity and trapezium distortion to name a few. This requires skill and patience normally beyond that of the average user/consumer. Additionally, PTV systems often need to be re-converged after shipping or major changes in climate.